(7) Project Summary/Abstract Broad Objectives: The proposed project aims to examine the associations between vagal tone and adolescents? functioning within close friendships. Given the implications of vagal tone and friendship support for the development of internalizing symptoms, the proposed project is expected to provide important information regarding psychological health in adolescence. The applicant?s career goals involve developing a program of research that examines how adolescents? affective, behavioral, and physiological responses to friend interactions interfere with or promote positive socioemotional adjustment. Specific Aims: Four specific aims are proposed. Using respiratory sinus arrhythmia (RSA) to index vagal tone, the proposed project will test resting RSA as a predictor of adolescent friends? behaviors during problem talk (Aim 1), examine bidirectional influences between adolescent friends? RSA reactivity and their problem talk behaviors (Aim 2), and model RSA coregulation between adolescent friends during problem talk and test links between behaviors during problem talk and the degree of coregulation (Aim 3). Moderators of the links between RSA and adolescent friends? behaviors during problem talk will also be tested (e.g., sex; Aim 4). Method: Adolescents and their same-sex friends (N = 200 adolescents; 100 dyads) will participate in a lab-based project. To ensure that participants do not have active psychopathology that could influence the measurement of RSA, participants will be screened for symptom levels that represent diagnosable disorders. Youth will complete brief survey measures regarding demographic and health information, problem severity, and emotional reactivity. Adolescents will be continuously monitored for heartrate and respiration during a resting task and an observed interaction in which they discuss a personal problem. Heartrate and respiration data will assess RSA (resting, reactivity, coregulation) and interactions will be coded for positive engaged and negative responses. Significance: The proposed project makes important conceptual contributions in that it will be the first to consider vagal tone within adolescent friendships and takes a unique, multi-method dyadic approach. Regarding practical contributions, a more nuanced understanding of the links between vagal tone (resting, reactivity, coregulation) and social functioning can identify specific physiological deficits to target in interventions. Further, examining specific behaviors that are linked to socioemotional adjustment provides behavioral targets for interventions. Training Goals: The applicant will develop expertise in the theoretical background, methodological approaches, and applications of autonomic physiological measures (Goal 1), build mastery in the study of coregulation (Goal 2), gain skills in statistical approaches to analyzing dyadic time series data appropriate for examining interpersonal patterns of change over time (Goal 3) and gain professional development skills (Goal 4). These goals will help the applicant to achieve her broader career goals of becoming an independent researcher with a unique, multi-faceted program of research in the field of developmental psychology.